Electrosurgery/electrocautery, employed during surgical procedures, creates high amplitude electrical artifacts during intraoperative biophysiological recordings. When displayed acoustically through a loud speaker, electrosurgery/electrocautery artifacts appear as a loud buzz or other annoying sound; and when displayed through an oscilloscope these artifacts clutter the recording. Since these artifacts provide no useful information, it is desirable to eliminate them from or minimize their effects in the recording. There are several prior art arrangements for eliminating electrosurgery/electrocautery artifacts from biophysiological recordings. Among these known arrangements are a footswitch which disables the signal path to the recorder. The footswitch may be independent of the electrosurgery apparatus or it may be included as a hardwired modification of the footswitch of the electrosurgery apparatus. In the case of an independent disable footswitch, a technician with responsibility for the recording and display of the biophysiological signals, operates the switch in anticipation of electrosurgery during a surgical procedure. Such an arrangement requires the technician to have special knowledge of surgical procedures and requires constant and careful observation of the surgery. While the integration of the footswitch into the electrosurgery apparatus is reliable and overcomes the shortcomings of the independent footswitch, the hardwired modification of all of the electrocautery units which may be employed during a surgical procedure is burdensome.
Radiofrequency antennas and/or "in-line" detection devices have been used to detect radiofrequency energy to trigger circuitry to disable recording. An example of such an arrangement is a cardiac doppler device used for intraoperative monitoring of heart functions and "air-embolus" monitoring.
In addition to automatic elimination of signals due to electrosurgery in the recording of biophysiological responses, it is desirable to eliminate the signals which are due to the electrical stimulation signals from the electrophysiological signals derived from a surgical patient. The recording of the electrical stimulation may interfere with the ability to record and/or interpret the intended biophysiological (true) response to the electrical stimulation. The temporal separation between the stimulus and the response and the use of fast recovery amplifiers may permit resolution of the stimulus and the response in oscillographic recorders. However, such resolution is not possible in the case of acoustic recorders. Stimulus artifact elimination has been achieved previously in the design of an acoustic EMG monitoring device e.g., Grass NL-1 Nerve Locator/Monitor. In that apparatus, a synchronous pulse from the electrical stimulus generator triggers a gate circuit which eliminates the period of electrical stimulus presentation from the recorded signal. A variable control allows the muted interval to be adjusted within predetermined limits.